1. Field of the Invention
This invention relates to a fuel staged burner for boilers and other process heating equipment such as hot water generators, steam flood heaters, fluid heaters, furnaces, radiant tubes, or kilns which are fueled by gaseous or liquid fuels, which burner is designed to reduce the formation of nitrogen oxides (NOx) simultaneously with complete combustion at low excess oxidant (overall stoichiometric ratios not exceeding 1.25). The burner provides fuel to the boilers and other process heating equipment in stages that are transversely oriented with respect to the center axis of the burner. This design results in lower levels of NOx in the flue gases than comparable burner designs without staging. This burner provides several advantages in comparison to burners that provide longitudinally oriented stages relative to the center axis of the burner, including the introduction of secondary or tertiary or quaternary fuel-oxidant mixtures at a lower temperature (not preheated), resulting in lower NOx levels during and after combustion; fewer apparatus components extending into the combustion chamber, resulting in lower manufacturing and maintenance costs; and avoidance of complex ducting and cooling means to avoid overheating of the staged fuel-oxidant mixtures.
2. Description of Related Art
Conventional combustion of fossil fuels produces elevated temperatures which promote complex chemical reactions between oxygen and nitrogen, forming various oxides of nitrogen as by-products of the combustion process. These oxides, containing nitrogen in different oxidation states, generally are grouped together under the single designation of NOx. Concern over the role of NOx and other combustion by-products, such as sulfur oxides, carbon monoxide, total hydrocarbons and carbon dioxide, in numerous environmental problems has generated considerable interest in reducing the formation of these environmentally harmful by-products of combustion.
Natural gas is a clean fuel which can help reduce these emissions. As a result, numerous ultra-low emission, natural gas-fired combustion systems are under development.
Known methods of combustion for reducing NOx emissions from combustion processes include flue gas recirculation and staged combustion. See, for example, U.S. Pat. No. 4,004,875 which teaches a low NOx burner for combustion of liquid and gaseous fuels in which the combustion area is divided into at least two stages and the combustion products are recirculated, cooled and reintroduced into the primary combustion zone, resulting in a reduction of NOx emissions. The secondary combustion air is introduced into a secondary combustion zone downstream of the primary combustion zone in an amount sufficient to complete combustion therein. The fuel and primary combustion air are introduced into a primary combustion zone formed by a burner tile which provides a high-temperature environment for the fuel and air mixture to promote combustion. Except for the opening into the secondary combustion zone, the burner tile is completely surrounded by a steel enclosure forming an annular space around the tile. Thus, as fuel and air are injected into the primary combustion zone, part of the partially combusted fuel and air is recirculated around the outside of the burner tile in the annular space between the tile and the steel enclosure and the back into the upstream end of the primary combustion zone.
U.S. Pat. No. 4,629,413 teaches a low NOx burner utilizing staged combustion in which a mixture of primary combustion air and fuel is introduced into a primary combustion chamber and secondary combustion air is introduced into the combustion chamber in a manner such that the mixing of the secondary combustion air with the flame generated by the mixture of fuel and primary combustion air is delayed. To further inhibit the formation of NOx emissions, cooled flue gases are recirculated within the combustion chamber into the fuel-rich combustion zone at the base of the flame, that is, the upstream end of the primary combustion zone.
U.S. Pat. No. 5,044,932 also teaches a process and apparatus for reducing the NOx content of flue gas effluent from a furnace in which cooled flue gases are internally recirculated from the downstream end of the combustion chamber into the upstream end of the combustion chamber where it undergoes reactions with the flame generated by the fuel and air introduced into the upstream end of the combustion chamber. Flue gas recirculation for mixing with primary combustion air and fuel prior to initiation of combustion is taught by U.S. Pat. No. 5,092,761.
A combustion process producing low NOx emissions utilizing staged combustion is taught by U.S. Pat. No. 4,007,001 in which 0-65% of the total air required for combustion is introduced into a primary combustion zone and 5-25% of the total air required for combustion is provided to a secondary combustion zone. Both U.S. Pat. No. 4,021,188 and U.S. Pat. No. 3,837,788 teaching staged combustion with less than a stoichiometric amount of air and primary combustion chamber, with additional air being added to the secondary combustion chamber for completion of combustion.
U.S. Pat. No. 4,575,332 teaches staged combustion in a swirl combustor with forced annular recycle of flue gases to the upstream end of the primary combustion zone, and U.S. Pat. No. 4,395,223 teaches staged combustion with excess air introduced into the primary combustion zone with additional fuel being introduced into the secondary combustion zone.
Temperature in the primary and secondary combustion zones of a combustion chamber is a critical parameter by which NOx emissions from a combustion process can be controlled. By providing less than the stoichiometric requirement of combustion air to the primary combustion zone as taught by the prior art, temperatures within the primary combustion zone are substantially below the temperatures of a primary combustion zone into which a stoichiometric, or more than a stoichiometric, requirement of air is introduced. However, the heat generated in the primary combustion zone in accordance with known combustion processes is conveyed into the secondary combustion zone into which secondary combustion air required for completing combustion of the fuel is introduced. Thus, the net heat within the combustion chamber remains unchanged.
Accordingly, it is one object of this invention to provide a combustion process which produces low pollutant emissions, in particular, low NOx emissions.
It is another object of this invention to provide a burner for staged combustion in which staging is carried out laterally or transversely, that is, distributed on a plane perpendicular or normal to the burner axis, also referred to herein as the center or central axis.
These and other objects of this invention are addressed by an apparatus comprising at least one wall enclosing a chamber and forming at least one fuel inlet opening, at least one oxidant inlet opening and a plurality of fuel/oxidant outlet openings. The plurality of fuel/oxidant outlet openings are formed by a portion of the wall disposed on a fuel/oxidant outlet side of the chamber and disposed at at least one radial distance from the center axis of the apparatus. A recirculation element, preferably in the form of a hollow cylinder or sleeve, is disposed on the fuel/oxidant outlet side of the chamber and is coaxially aligned with the center axis. The recirculation sleeve comprises a combustion products inlet end and a recirculated combustion products outlet end with the recirculated combustion products outlet end oriented in the direction of the at least one wall and disposed at a distance therefrom. Disposed within the chamber are a plurality of fuel distributors, each of which has a fuel inlet and a plurality of fuel outlets. Each of the fuel outlets is aligned with a corresponding fuel/oxidant outlet opening. As will be discussed in more detail hereinbelow, each fuel/oxidant outlet opening corresponds to a combustion stage produced by the apparatus.
The objects of this invention are further addressed by a method for combustion of a fuel in which a plurality of fuel streams are introduced into a combustion chamber, with each of the fuel streams penetrating at one of at least two different axial lengths into the combustion chamber. Each axial length corresponds to a fuel stage. An oxidant is introduced into the combustion chamber and the fuel is ignited, resulting in formation of a flame and combustion products. At least a portion of the combustion products is recirculated to a base region of the flame.